Carbon fiber is ⅕ lighter, but at least 10 times stronger than steel. Thus, carbon fibers are being used for high strength structural materials in a variety of industrial fields such as including aerospace, sports, automobiles, and bridges. With the rapid development and high tech of the automotive and aerospace industry, carbon fibers are receiving much attention as a next-generation material, and especially in the automotive industry, with the movement towards environment friendly, low energy consuming future automobiles, carbon fibers are in increasing demand. Also, with the growing demand for lighter automobiles as well as environmental regulations relating to automobile exhaust gas at future issue in the automotive industry, carbon fiber-reinforced composites that can reduce the weight of automobiles while maintaining the structural and mechanical strength are in increasingly high demand.
However, carbon fibers are now too expensive to be used for the above purposes. For a wide range of applications in the automotive industry and in the field of construction and infrastructures, carbon fibers need to have mechanical properties suitable for use in each industry at reduced costs that are as low as at least ⅓ of the present level.
Generally, carbon fibers are prepared through an oxidation⋅stabilization process for performing oxidation and stabilization by applying heat in the oxidizing atmosphere to make precursor fibers unfusible, and a carbonization process for carbonizing the oxidized⋅stabilized fibers at high temperature. Subsequently, a graphitization process may be performed. In this instance, the precursor fibers of the carbon fibers include polyacrylonitrile (PAN), pitch, rayon, lignin and polyethylene. Among them, the polyacrylonitrile (PAN) fibers are an optimum precursor for preparing high performance carbon fibers as compared to the other precursors because of having a high carbon yield of 50% or more and a high melting point. Accordingly, most of carbon fibers are currently prepared from polyacrylonitrile fibers.
The polyacrylonitrile fibers for carbon fiber precursors are made from a copolymer containing about 95 wt % of acrylic monomers (acrylonitrile; AN) and about 5 wt % or less of acrylic comonomers with a carboxyl functional group such as itaconic acid that serves as a catalyst in the stabilization reaction. The polyacrylonitrile fibers allow for preparation of carbon fibers having high performance.
However, the cost of polyacrylonitrile fibers for carbon fiber precursors is much higher than general fibers. Generally, a precursor fiber is given the weight of 43%, an oxidation⋅stabilization process is given the weight of 18%, a carbonization process is given the weight of 13%, and a graphitization process is given the weight of 15% in the cost of a carbon fiber. Accordingly, not only precursor fiber cost reduction but also an oxidation⋅stabilization process may be a key technology in the carbon fiber cost reduction technology. An oxidation⋅stabilization process is a very slow reaction compared to a carbonization process, and consumes a largest amount of energy in the carbon fiber preparation process.
The oxidation⋅stabilization process is a process which reacts fibers with oxygen to cause a dehydrogenation reaction and a cyclization reaction so that the molecular structure of the fibers is made more stable, and the oxidation⋅stabilization process using heat occupies most of the total process time in the carbon fiber preparation process, and thus, a variety of attempts have been made to reduce the stabilization process time.
Instead of the thermal stabilization process, a plasma generated using RF, DC, microwave or pulsed power may be used, allowing oxygen molecules reacting with fibers to be converted to highly reactive oxygen species (oxygen atom, ozone, NxOy, etc.), and through this, many studies have been made to increase the reaction speed of oxygen reacting with fibers to achieve a fast reaction.
However, when fibers have many bundles, it is difficult that heat or oxygen species penetrate deep in the bundles enough to cause a reaction in a general thermal stabilization process or an oxidation⋅stabilization process using plasma, and inner fiber strands are not fully stabilized as opposed to outer fiber strands, and in this case, carbon fibers formed after a carbonization process have notably low strength, resulting in overall quality degradation.
Recently, much attention is paid to polymer modification such as polymer crosslinking using electron beam and reactive group introduction, and electron beam irradiation causes a variety of polymer structure changes such as polymer crosslinking, breaking in polymer chains, reactive group introduction, and crystallinity change. When electron beam irradiation is applied to polyacrylonitrile fibers, a crosslinked bond is formed between polymer chains by carbon radicals generated at the polymer chains, and some is applied to nitrile groups at the side chain to generate imine groups. Thus, it is more efficient and environment friendly than a thermal process or a radiation process such as gamma rays and ultraviolet rays. When electron beam irradiation is applied to polyacrylonitrile fibers, the electron beam penetrates to a few centimeters depth and causes crosslinking, and its advantage is that even large-tow fibers are uniformly crosslinked. For this reason, attempts were made to use an electron beam for oxidation⋅stabilization of polyacrylonitrile fibers, but it is known that an electron beam only achieves crosslinking and does not cause a cyclization reaction by nitrile groups of polyacrylonitrile fibers.
Generally, defects and structural morphology of fibers are factors that restrict the tensile strength of carbon fibers, and to overcome the problem, a variety of stabilization and carbonization methods have been proposed, and one of them, gamma ray radiation is known to increase the strength of carbon fibers. The tensile strength and the modulus of elasticity of carbon nanotubes is 23˜63 GPa and 640˜1060 GPa, respectively, while the highest level of tensile strength and modulus of elasticity in existing carbon fibers is in the range of 6˜7 GPa and 300˜320 GPa. When considering that the theoretical carbon fiber tensile strength is 100˜150 GPa, increasing the tensile strength of carbon fibers is a challenge. Thus, there are attempts to prepare carbon fibers by incorporating carbon nanotubes (CNTs) having excellent tensile strength and elasticity into composite structure. As carbon fiber precursors, nanocarbon composite PAN fibers were prepared by various methods such as including wet spinning and dry jet wet spinning. To enhance the mechanical properties of a polymer including polyacrylonitrile, improve electrical conductivity, or give functionality like electrostatic fibers, fibers based on composites of nanocarbons such as carbon nanotubes (Reference: RU 2534779 C1, CN 101619509 A, CN 101250770 A; Polymer, 48, (2007) 3781, Carbon, 77 (2014) 442] and graphene [Reference: CN 102586951A, CN 102534870 A] were prepared and attempts have been made to carbonize the nanocarbon composite polyacrylonitrile fibers to prepare carbon fibers with superior properties.
The introduction of carbon nanotubes (CNTs) and graphene to polymer fibers allows for the preparation of polymer grafted nanocarbon by synthesis of polyacrylonitrile polymer in the presence of CNT, or carbon fibers having improved properties by stabilization and carbonization of nanocarbon composite polyacrylonitrile fibers prepared by spinning a solution prepared by dissolving polyacrylonitrile in a nanocarbon dispersion. However, the tensile strength of carbon nanotube composite carbon fibers has a considerable level of improvement in the properties such as the tensile strength on the general purpose carbon fiber level with the addition of carbon nanotubes, while the effect on high performance carbon fibers is not yet known. In the preparation process of the strength nanocarbon composite polyacrylonitrile precursor fibers with high strength, a high ratio stretching process has a high risk of creating a defect structure such as the release at the interface between polyacrylonitrile and CNT in the composite polyacrylonitrile fibers. Thus, attempts were made to solve the problem by imparting chemical coupling using polyacrylonitrile polymer-grafted CNT, but the interface problem between CNT and polyacrylonitrile is still a challenge for improving the mechanical properties of carbon fibers.
On the other hand, electron beam irradiation enables polymer crosslinking or breaks polymer chains, and is used for modification of polymers and composites, but when applied to nanocarbon such as carbon nanotube and graphene, electron beam irradiation generates a new covalent bond between carbon nanotubes and consequently increases the strength [Reference: AIP Conference Proceedings (2004), 723, 107], and when high energy beam irradiation is applied, carbon nano materials produce heat, and taking advantage of this, it is used for graphitization reaction. When electron beam irradiation is applied to a carbon nanotube sheet including carbon nanotube and a crosslinker, the strength of the carbon nanotube sheet may be improved. This is because the crosslinker added forms a crosslinking structure between carbon nanotubes and between carbon nanotube bundles by the application of the electron beam. That is, it is known that a carbon-carbon single bond is efficiently formed between carbon nanotubes and between carbon nanotube bundles by electron beam irradiation, ensuring high strength.
Attempts were made to oxidize⋅stabilize carbon nanotube containing carbon fiber precursor fibers by electron beam irradiation solely. That is, when oxidation⋅stabilization is to be performed by the heat from carbon nanotube by electron beam irradiation, electron beam irradiation usually induces only a crosslinking bond between polymer chains, so the heat from carbon nanotube alone is insufficient for completing a cyclization reaction of —CN groups. Also, when oxidation⋅stabilization is to be performed by electron beam irradiation under temperature atmosphere heated by excessive electron beam irradiation, a crosslinking reaction by electron beam irradiation is as short as a few minutes or less, resulting in a very insufficient cyclization reaction. If the electron beam irradiation time increases and a crosslinking bond between polymer chains excessively increases, rather the crosslinked structure impedes a cyclization reaction by —CN groups, failing to complete the cyclization reaction and consequently oxidation⋅stabilization reaction, resulting in property degradation of carbon fibers.